Abstract: The iron–chromium redox flow battery (ICRFB), widely recognized as the first true redox flow battery (RFB), exhibits distinct advantages over conventional lithium-based energy storage systems, including enhanced safety, exceptionally long cycle life, flexible structural design, and low operational and maintenance costs. Consequently, ICRFBs have emerged as a pivotal research focus for large-scale robust energy storage applications. In the ICRFB system, low-cost and abundant iron and chromium chlorides serve as redox-active materials, rendering it one of the most cost-effective energy storage systems available. The core components of an ICRFB include electrolytes, membranes, and electrodes. Among these, the electrode plays a critical role by facilitating electron transport, ion migration, and catalytic reactions within the battery system. The microstructure and catalytic performance of the electrode critically influence the reaction kinetics of the iron and chromium active species, as well as the charge–discharge efficiency of the battery. Commonly used electrode materials in ICRFBs include carbon-based materials, porous metals, and conductive polymers. Among them, carbon-based electrodes dominate owing to their low cost, excellent chemical stability, three-dimensional conductive network, and long-term stability under acidic conditions. Nevertheless, carbon-based electrodes have several limitations, such as insufficient active sites, low specific surface area, and poor electrolyte wettability, which hinder further improvements in electrochemical performance. To address the existing technical bottlenecks, this review systematically summarizes recent advancements in multi-scale modification strategies for developing high-performance carbon electrodes. Surface modifications of carbon-based electrodes primarily fall into two categories. The first category involves introducing oxygen-containing functional groups (e.g., hydroxyl and carboxyl groups) onto the electrode surface through methods such as heat treatment, acid treatment, electrochemical activation, steam treatment, plasma treatment, and microwave etching. These modifications regulate the surface chemistry, enhance electrode hydrophilicity, improve catalytic activity, and suppress hydrogen evolution side reactions. The second category involves loading catalysts—classified into metallic elements, metal compounds, and non-metallic materials—onto the electrode surface to construct catalytically active sites. This approach increases the specific surface area of the electrode and enhances the adsorption capacity of active species, thereby lowering reaction energy barriers and accelerating charge transfer. Such modifications effectively enhance the catalytic activity of the carbon electrode and the reaction kinetics of active species, thereby improving the overall battery performance. Although surface modification of carbon electrodes has shown significant advancements in enhancing their performance, a comprehensive understanding of the underlying mechanisms is still lacking. Several aspects, including the “functional group–active site structure–activity relationship” and “catalyst–interface charge transfer pathway,” remain underexplored. This review focuses on the existing multi-scale surface engineering methods for carbon-based electrodes, providing a detailed analysis of the underlying mechanisms behind different modification methods. Furthermore, it introduces high-throughput computational methods to establish quantitative models linking modification strategies to performance improvements. This work aims to provide a theoretical basis for overcoming the “activity?stability?cost” trade-off of carbon-based electrodes and to offer guidance for designing electrode materials for use in next-generation high-performance flow battery electrodes.